Origins of Oil and Gas Sector Methane Emissions: On-Site Investigations of Aerial Measured Sources

Success in reducing oil and gas sector methane emissions is contingent on understanding the sources driving emissions, associated options for mitigation, and the effectiveness of regulations in achieving intended outcomes. This study combines high-resolution, high-sensitivity aerial survey data with subsequent on-site investigations of detected sources to examine these points. Measurements were performed in British Columbia, Canada, an active oil- and gas-producing province with modern methane regulations featuring mandatory three times per year leak detection and repair (LDAR) surveys at most facilities. Derived emission factors enabled by source attribution show that significant methane emissions persist under this regulatory framework, dominated by (i) combustion slip (compressor exhaust and also catalytic heaters, which are not covered in current regulations), (ii) intentional venting (uncontrolled tanks, vent stacks or intentionally unlit flares, and uncontrolled compressors), and (iii) unintentional venting (controlled tanks, unintentionally unlit/blown out flares, and abnormally operating pneumatics). Although the detailed analysis shows mitigation options exist for all sources, the importance of combustion slip and the persistently large methane contributions from controlled tanks and unlit flares demonstrate the limits of current LDAR programs and the critical need for additional monitoring and verification if regulations are to have the intended impacts, and reduction targets of 75% and greater are to be met.


S1 Study Region
shows the locations of the aerial survey sites within the province of British Columbia, Canada and identifies sites with detected emissions (via airborne gas mapping LiDAR) and those investigated during follow-up visits by ground technicians using optical gas imaging cameras. These aerial survey sites are also considered in parallel work developing and demonstrating a protocol to produce measurement-based methane inventories 1 .

S2 Greenhouse Gas Emissions of Flaring Versus Venting
Because the radiative forcing of fossil methane is approximately 82.5/29.8 times greater than carbon dioxide on a mass-basis when evaluated on a 20/100 year time horizon 2 , combustion of methane to carbon dioxide S3 will always reduce equivalent greenhouse gas (GHG) emissions relative to venting. However, for industrial flare systems where additional fuel may be required for pilot and purge systems to keep the flare operating and available, it is prudent to consider the potential added GHG emissions of these flows. Notably, continuous flare systems generally do not require purge gas 3 ; it is possible to significantly reduce or eliminate purge flow requirements on intermittent flares through the use of seals [3][4][5] or flame arrestors 6 ; and/or associated GHG emissions can be avoided through the use of nitrogen purge systems 4,5 . Similarly pilot flames may not be required for flares using electronic ignition systems. Nevertheless, in the most conservative case, maintaining an operating flare system to handle intermittent gas releases would incur GHG emissions from both purge and pilot flows. These GHG emissions can be calculated assuming recommended  Table 6 in Tyner and Johnson 7 (i.e., 91.87% CH4, 2.84% C2H6, 1.27% C3H8, 0.62% C4H10, 0.22% C5H12, 0.08% C6H14, 0.09% C7H16, 0.73% CO2, plus other non-carbon, non-GHG species), GWPCH4=82.5 (as relevant for near-term 2025, 2030, and 2050 targets), and conservatively assuming that both the pilot and purge gas burn with 98% carbon conversion efficiency, then continuous operation of pilot and purge would produce 6.91 kgCO2e/h. Consider then a vented gas source of 104.09 m 3 /month (whole gas) with the same composition such that it includes 0.089 kgCH4/h of methane. Flaring this gas at 98% carbon conversion efficiency would produce 0.424 kgCO2e/h (calculated assuming the emissions are driven by stripping of unburned fuel 8,9 ).
Conservatively assuming the flare still requires the full 6.91 kgCO2e/h for pilot and purge, this equates to total GHG emissions of 7.33 kgCO2e/h. Direct venting of this gas would produce the same 7.33 kgCO2e/h. Thus, allowing for pilot and purge flow, flaring will reduce GHG emissions of any vented gas stream greater than 104.09 m 3 /month (whole gas) or methane source greater than 0.089 kgCH4/h. Repeating these calculations assuming GWPCH4=29.8 suggests flaring will reduce GHG emissions of any vented gas stream greater than S4 241.6 m 3 /month (whole gas) or methane source greater than 0.206 kgCH4/h. A similar calculation is included in CSA Standard Z620.3:22 10 .
As shown in Table S1 below, for each measured unlit flare or intentional vent stack, it is possible to calculate the maximum duration of that vent beyond which net greenhouse gas emissions would be lower via flaring even if it meant operating a 4-inch diameter flare continuously on standby for a month at recommended maximum pilot and purge flow rates. This is done by iteratively calculating the vent duration such that the vented gas GHG emissions match the GHG emissions incurred from flaring that same volume of gas plus the GHG emissions from maintaining an operating 4-inch diameter flare at maximum recommended purge and pilot flow rates for a month. Notably, if there is already an operating flare on site, or a combustor or other combustion device is used that does not require pilot and purge flows, then GHG emissions would always be reduced by burning the vent gas, no matter the duration of the venting event. Continuous flare Lit and emitting during both flight and reflight 3.5 95.9 n/a a Hours at measured venting rate beyond which net greenhouse gas emissions would be lower via flaring even if operating a 4-inch diameter flare continuously on standby for a month at recommended maximum pilot and purge flow rates. b Ground team noted that this flare "is only used as a vent stack and is continuously venting". c Ground team noted this flare is being used as a vent stack. d This flare was surveyed in 2019 11 and observed to be lit without detected emissions. e This flare was surveyed in 2019 11 and visibly unlit and emitting methane at 5.0 kg/h. However, ground team noted that the pilot flame was operating and visible in their subsequent visit after the flights in 2021. f This vent stack was surveyed in 2019 11 and not detected. g Based on measured emission rate of 52.4 kg/h during initial flight. h Based on measured emission rate of 4.0 kg/h during reflight. i This is reportedly a legacy facility as pit flares have not been approved for several years and may only be constructed if "specifically authorized" 12 . S6 Figure S2. Distribution of total measured tank emissions at individual sites and overlaid with forthcoming regulated limit for existing tanks of 9000 m3/mo (whole gas), which is converted to a methane emission rate of 7.36 kg/h assuming a representative methane fraction of 88%.

S3 Total Site Tank Emissions
As noted in the main text, half (52%) of sites in the survey had aggregate measured tank emissions above the upcoming limit of 9000 m 3 /mo (whole gas, equivalent to 7.36 kg/h of methane), and these sites represent 90% of the total measured methane from tanks. In principle, bringing these sites into compliance with the limit could reduce emissions by 70%. Unfortunately, closer analysis of the data suggests this is not realistic given that approximately 65% of these excess tank emissions are from tanks that already have controls and are currently subject to regulated three times per year leak detection and repair (LDAR) surveys. Ideally, if regulations were working as intended, these emissions should already be reduced toward zero. Moreover, if the current measured population average for controlled tanks is taken as the achievable limit without additional MRV, then the upcoming tank limit is only likely to net a reduction of 27%.    Table S6: Partial sample of supplement pneumatic inventory data at separators and other sources (see Figure 5 in the main text) from ground follow-ups. Complete data are provided in separate .xlsx file.